The use of ionising radiation in medical imaging

Ionising radiation may be defined as energy in transit from one site to another (Sherer et al., 2006: 8). Types of ionising radiation are x-radiation and gamma radiation.

Ionisation is the process when electromagnetic radiation has a high enough frequency to transfer enough energy to the electrons to remove them from the atoms to which they were attached. Ionisation is the basis of the interactions of X-rays with human tissue.

Ionisation is beneficial for producing X-ray images, but has the undesirable effect of possibly producing some harm in biological material (Sherer et al., 2006: 10). For the purpose of this study, only ionising radiation was dealt with, specifically X-rays. The latter were discovered by Roentgen in 1895 and represent a form of ionising radiation that produces positively and negatively charged ions when passing through matter. The production of these ions is the incident that may cause injury in normal biological tissue (Sherer et al., 2006: 2). X-rays are high energy photons and are produced artificially by the rapid slowing down of an electron beam. X-rays are similarly penetrating, and in the absence of shielding by dense materials, can deliver significant doses to internal organs (International Atomic Energy Agency, 2004: 8).

X-rays have the following properties.

 They can affect photographic plates.

 They are not affected by magnetic or electrical fields.

 They can cause some materials to fluoresce.

 They can cause ionisation.

 They can be absorbed by elements that have a high atomic number i.e. lead.

 They can penetrate most materials including soft tissues and bones (Seeram, 1997: 288).

The quantity of an X-ray beam is defined as the number of x-ray photons in the beam per unit of energy; the quality of the X-ray beam refers to the penetrating power of the x- ray beam (Seeram, 1997: 288).

These are some of the factors that influence radiation dose to a patient. Radiation dose may be defined as the amount of energy transferred to electrons by ionising radiation (Sherer et al., 2006: 10).

A radiographic image is produced as follows. The primary beam is attenuated by a patient‟s tissues; the exit or remnant radiation is composed of variable intensities; the image receptor then receives or captures the exit or remnant radiation and produces a latent or invisible image that requires processing. Image formation for DR differs from the image formation for film-screen radiography, since image receptors respond differently to the exit or remnant radiation. In DR, the digital image is stored and displayed as computer data visible on a monitor as a range of brightness levels, whereas in film-screen radiography, the film image is processed to display a range of densities on a polyester sheet (Terri and Fauber, 2014: online).

Image acquisition for film-screen radiography is as follows. Film is used to acquire, process, and display the radiographic image. An emulsion layer is adhered to a polyester sheet and serves as the radiation-sensitive and light sensitive layer of the film.

Prior to the introduction of intensifying screens, a film was exposed to the primary beam and then processed to produce the required image. In an attempt to decrease the radiation exposure to a patient, intensifying screens were developed. A film is placed in a light-tight cassette that contains two intensifying screens. The film is then exposed to the light emitted from the intensifying screens in proportion to the amount of radiation exposure (Terri and Fauber, 2014: online).

Image acquisition for CR is as follows. The exit or remnant radiation interacts with the imaging plate (IP), which is composed of barium fluoride bromide crystals and is coated with europium; absorbed energy is then stored in the photostimulable phosphor material; some energy is released as visible light, but most result in electrons being released in the phosphor layer due to photoelectric interactions; electrons are then trapped in the phosphor layer until light energy is released during laser scanning (Terri and Fauber, 2014: online).

The CR latent image is digitised in three stages: scanning; sampling, and quantisation.

DR is subdivided into direct and indirect conversion. Image acquisition for the former is as follows. The flat panel detector array is exposed to the exit or remnant radiation; the scintillator converts X-rays into light; the light energy is transformed into electric signals and then the electric signals are digitised. Image acquisition for direct conversion is as follows. The flat panel detector array is exposed to the exit or remnant radiation;

selenium converts X-rays into electric signals and then the electric signals are digitised (Terri and Fauber, 2014: online).

The basis of the radiography profession is the creation of good quality radiographic images and patient radiation protection practices. In radiography it is essential for radiographers to comprehend the principles of electromagnetic radiation, and to effectively apply exposure techniques. Radiographers administer radiation so that patient radiation exposure is assumed safe and that occupational risk is the same as for most safe professions. In this age of technology many things are rapidly being modified in the radiology imaging speciality; nevertheless, the physical ideas of radiation exposure to produce images of diagnostic quality remain an essential part of the imaging discipline. In addition, an X-ray tube design is engineered with patient radiation protection as one of its highest criteria. Similarly, a radiographer's selection of radiographic techniques includes the highest radiation protection principles possible.

Principles of radiation exposure that reduce unnecessary exposure and emphasise imaging benefits are foundation principles of the radiography profession (CEEssentials, 2018: online).

To ensure that DR is practiced safely and correctly, the Directorate Radiation Control compiled a code that sets standards and requirements for radiation safety associated with the use of medical diagnostic x-ray equipment (DoH, 2012: 3). Some of the regulations promulgated by them are discussed below.